1. Field of the Invention
The present invention relates to a printing apparatus wherein a printing unit, in which a plurality of printing elements are arranged, provides a printing agent for a printing medium to form an image. The present invention relates particularly to a method and a configuration for adjusting the displacement of print position for printing elements.
2. Description of the Related Art
Printing apparatuses having some function of a printer, a copiers and a facsimile, or printing apparatuses employed as the output apparatuses for multifunctional electronic apparatuses that include a computer or a word processor and for workstations employ image information (including, for example, character information) to print images (including characters and like) on printing media, such as paper and plastic thin sheets. The printing methods of the printing apparatuses can be categorized as being inkjet, wire dot, thermal and laser beam types. The printing apparatus of an inkjet type (an inkjet printing apparatus) permits a printing unit (printing head) to eject ink onto a printing medium for printing images. Compared with other printing types, inkjet printing apparatuses have several superior features, in that high definition printing is easily provided and fast printing is performed in a superior, noiseless state, and in that such apparatuses are inexpensive. Therefore, the employment of inkjet printing apparatuses has spread and currently covers a wide range, from office use to personal use.
Generally, inkjet printing apparatuses employ printing heads wherein ink discharge ports and a plurality of printing elements, including liquid paths for supplying ink to the ink discharge ports, are arranged. Further, in order to cope with color printing, inkjet printing apparatuses are frequently equipped with printing heads for multiple colors.
Generally, inkjet printing apparatuses are categorized as being either serial printing types or line printing types, depending on the different printing operations that can be performed. To form an image, a serial type printing apparatus intermittently repeats a main scanning operation, according to which the printing head is moved relative to a printing medium to form an image, and a sub-scanning operation, according to which a printing medium is conveyed in a direction perpendicular to the main scanning direction. Whereas for a line type printing apparatus, an immobile printing head is used, wherein multiple printing elements are arranged in consonance with a printing width for a printing medium. To form an image, while the printing head performing print operation, the printing medium is moved at a predetermined speed in a direction different from the direction in which the printing elements are arranged.
Although the line type printing apparatus can perform printing rapidly, such an apparatus tends to be large. On the other hand, the serial type printing apparatus employs a small printing head and is to cope with various printing media sizes. When the number of times scanning is performed, or the main scanning direction relative to an image area is changed, various printing speeds and image qualities can be provided that are in consonance with the desires of a user. Therefore, recently, widespread use is being made of the serial type inkjet printing apparatus, especially for the personal use.
However, the serial type inkjet printing apparatus also has an inherent problem.
FIGS. 1A and 1B are schematic diagrams for explaining an example manufacturing error for a printing head that is to be mounted on an inkjet printing apparatus. In FIGS. 1A and 1B, a printing head 1401 is formed by adhering to the printing head 1401 a plurality of chips 1402 that include a plurality of discharge ports 1403 for discharging ink. In the example in FIG. 1A, an ideal printing head manufactured with no error is shown. In the example in FIG. 1B, the chips 1402 are obliquely adhered to the printing head 1401. To perform printing, in accordance with an image signal, the individual discharge ports 1403 of the printing head 1401 discharge ink at a predetermined frequency, and are moved at a constant speed in the main scanning direction shown in FIGS. 1A and 1B. When one scanning is completed, a printing medium is conveyed in the sub-scanning direction shown in FIGS. 1A and 1B a distance equivalent to the printing width of the printing head.
FIGS. 2A and 2B are diagrams for explaining a printed image problem that is encountered when the printing head having the above described tilt is employed. In this case, the printing of ruled lines extended in the sub-scanning direction is shown. When discharge ports 1403 arranged on a printing head 1401 are not tilted, and when printing has been normally performed, straight ruled lines extended in the sub-scanning direction, as shown in FIG. 2A, are printed. On the other hand, when the individual chips 1402 are tilted as shown in FIG. 1B, ruled lines are obliquely printed during scanning, and are split into segments at the seams of the lines when sequential scanning is performed.
This occurrence of this phenomenon not, however, due only to the tilting of the printing head relative to the main body of the printing apparatus. During the manufacture of a printing head and a printing apparatus, specific errors must be taken into account, but various other discrepancies are also often found in the print results. The phenomenon shown in FIG. 2B, wherein ruled lines are tilted, can also occur due to various other factors, such as the inclination of the discharge face of the printing head relative to the face of a printing medium, the inclination of the printing element arrays on the printing head and variances in the discharge speeds of ink droplets discharged from the individual printing element.
FIGS. 3A to 3C are schematic diagrams for explaining an example wherein the shifting of a ruled line, as shown in FIG. 2B, occurs due to the inclination of the discharge port face of a printing head relative to the face of a printing medium. In the state in FIG. 3A, a printing head 1401 is not tilted relative to a printing medium 1503. In FIG. 3A, a carriage 1506, on which the printing head 1401 is mounted, is moved vertically to this drawing, relative to the printing medium 1503, along a carriage shaft 1507. As the carriage 1506 is moved, the printing head 1401 discharges ink droplets at a constant timing. While referring to the state shown in FIG. 3A, since the discharge port face of the printing head 1401 is parallel to the printing medium 1503, the distance between the printing medium 1503 and each of the discharge ports arranged on the discharge port face is identical. Thus, ink droplets discharged at the same timing land on to the printing medium 1503 at substantially the same time. That is, in FIG. 3A, both an ink droplet 1501, discharged from the leftmost discharge port, and an ink droplet 1502, discharged from the rightmost discharge port, land on the printing medium 1503 at almost the same time, and as indicated by a broken line 1504 in FIG. 3C, a ruled line parallel to the sub-scanning direction is printed.
In the state in FIG. 3B, the attached carriage 1506 is tilted at the carriage shaft 1507. In this case, the discharge port face of the printing head is also inclined relative to the printing medium 1503, and the distance between the printing medium 1503 and each of the discharge ports arranged on the discharge port face differs. That is, in FIG. 3B, an ink droplet 1501 discharged from the leftmost discharge port lands on the printing medium 1503 later than an ink droplet 1502 discharged from the rightmost discharge port. Since the carriage 1506 is moved vertically relative to this drawing, during the discharge operation, the differences in the landing timing appear as an inclination indicated by a broken line 1505 in FIG. 3C, just as if the discharge port array were tilted.
FIG. 4 is a schematic diagram for explaining an example wherein shifting within a ruled line, as shown in FIG. 2B, occurs due to the speed at which ink is discharged from individual discharge ports. A plurality of printing elements are arranged in a printing head under the same conditions; however, specific discrepancies may be present in the consumption of the driving power by the individual printing elements, and in members provided in the printing elements. These discrepancies may appear as differences in the discharge speed when ink is discharged from the discharge ports of the printing elements. In FIG. 4, the speed at which an ink droplet 1601 is discharged from the leftmost discharge port is the lowest, and gradually increases, discharge port by discharge port to the right. In this case, the ink droplet 1601, discharged from the leftmost discharge port, lands on a printing medium 1503 later than an ink droplet 1602, discharged from the rightmost discharge port. Since a carriage 1401 is moved vertically relative to this drawing, during the discharge operation, the difference in the landing timing appears as a difference, as indicated by the broken line 1505 in FIG. 3C, just as if the discharge port array were tilted.
As described above, and as shown in FIGS. 1A to 4, a printed ruled line can be tilted due to various reasons. Further, such differences in the print positions constitute a new image barrier for various situations, in addition to the printing of the ruled lines shown in FIG. 2B.
For example, for the serial type inkjet printing apparatus, a multi-pass printing method is sometimes employed in order to reduce a seam that appears at each scanning, or to reduce an uneven density that is caused by variances in the manufacture of discharge ports. According to the multi-pass printing method, data to be printed in one image area is divided into a plurality of patterns that are in complementary relationship each other, and an image is formed step by step by performing a plurality of scans. Since before and after each scan a printing medium is conveyed a distance that is shorter than the printing width of a printing head, a line is formed in the main scanning direction by a plurality of types of printing elements. Thus, the printing characteristics of the individual printing elements are dispersed across the entire image, and the entire image is smoothed.
However, when the above described tilt is present, even though the multi-pass printing method is employed, a displacement may occur in dot position to be complemented each scan, and a new image barrier, such as improper texture, may be the result.
Furthermore, for a printing apparatus wherein a plurality of printing heads are arranged for printing a color image, the degree of tilt may differ, depending on the printing heads. Because of this, color would be applied unevenly, or granularity deterioration (visual roughness of grains) would occur.
As described above, an image barrier due to tilting is a conventionally important problem for a serial inkjet printing apparatus. To resolve the problem occurring due to tilting, several measures have been proposed and applied.
For example, in Japanese Patent Application Laid-open No. 7-309007 (1995), disclosed is an inkjet printing system that includes an error correction circuit that adds an offset to image data printed by individual discharge ports, so as to reduce a print position error that is caused by the rotation of a printing head. Further, in Japanese Patent Application Laid-open No. 7-040551 (1995), an inkjet printing apparatus is disclosed wherein a plurality of discharge ports arranged in a printing head are divided into a plurality of blocks, and the order in which ink is discharged from the blocks and the discharge interval are controlled in accordance with the tilting of a printing head. In addition, in Japanese Patent Application Laid-open No. 11-240143 (1999), in order to correct a shift in print positions that occurs at a seam for scans, due to the tilting of a printing head, disclosed is a method whereby an offset value is designated based on a difference between a print position for the topmost discharge port and a print position for the lowermost discharge port, and whereby, through part of the discharge ports, data are printed by being shifted a distance equivalent to the offset value. Moreover, in Japanese Patent Application Laid-open No. 2004-009489, disclosed is an inkjet printing apparatus that includes means for changing, in accordance with the tilt of a printing head, the allocation of print data to individual discharge ports.
However, by using the methods described in the patent documents described above, wither the above described problems can not be satisfactorily resolved or a new another problem has arisen, and practical tilting correction is not ensured.
For example, in order to correct an error due to a tilt, in Japanese Patent Application Laid-open No. 7-309007 (1995), an inkjet printing system is disclosed wherein a printing head is divided into two or more nozzle groups (discharge port groups), and wherein the second nozzle group is offset relative to the first nozzle group, i.e., the timing is adjusted to perform printing. To provide the offset, a “method for generating a drive signal for discharging ink from the second nozzle group later (or earlier) than that for the first nozzle group” and a method “for generating data to be printed by the second nozzle group while an address is shifted” are disclosed in the embodiment. However, according to the first method, since drive signal transmission means for the individual nozzle groups is required, costs are increased, and the limit “the maximum value for a shift is dependent on the input of a data signal for an adjacent pixel” is also additionally provided. According to the second method, no limitations are imposed for the shift, however, when an accurate correction is to be performed, a higher resolution must be set for data to be printed, and the amount of image data required would be enormous.
In Japanese Patent Application Laid-open No. 7-040551 (1995), a method is disclosed whereby, by employing the fact that nozzles of one nozzle array are divided into a plurality of blocks for discharging ink, the order driving blocks are arranged in accordance with a tilt, and the intervals for the driving blocks are changed. However, according to this method, as well as the method disclosed in Japanese Patent Laid-Open Publication No. 7-309007, the limitation that “the maximum value for a shift is dependant on the input of a data signal for an adjacent pixel” is additionally provided. Therefore, the correctable range is limited to only a single pixel.
In Japanese Patent Application Laid-open No. 11-240143 (1999), a method is disclosed, whereby, between scans, the level of a tilt is identified based on the difference between the print positions of the distal end nozzle and the rear end nozzle, and in accordance with the difference, data is printed through part of the nozzles, while offset. Furthermore, in Japanese Patent Application Laid-open No. 2004-009489, a method is disclosed whereby data to be allocated to individual nozzles vary in accordance with the tilting level of a printing head. However, using either of the methods disclosed in Japanese Patent Application Laid-open Nos. 11-240143 (1999) and 2004-009489, corrections can only be performed at accuracy equal to a unit of a single pixel.
That is, when any methods disclosed in the above described patent documents are employed, direct means for correcting the tilt either adjusts, within one pixel, a timing for driving individual printing elements (nozzles), or shifts, by one pixel unit, the address of data to be printed. The first means can not cope with a displacement of one pixel or greater, and the second means can correct the tilt with an accuracy equal only to a single pixel unit. For the second means, the method for increasing the printing resolution may be employed to reduce the size of a single pixel. However, when the printing resolution is increased and exceeds a requested image quality, the volume of the image data required would be expanded, the printing speed would be reduced, and the cost of the printing apparatus would be increased. Thus, this is not a practical method.